A scanning apparatus for fluorescent optical storage reading is described in U.S. Pat. No. 6,009,065. Information carriers are organized in the form of a multilayer optical disc. The information is deposited or recorded as a sequence of fluorescent and non-fluorescent cells, the fluorescent cells being made of a material capable of generating an excited radiation when interacting with an exciting beam. The layers of the carrier are separated by thick layers, which are transparent to the wavelengths of the exciting beam and the excited radiation. Such a multilayer optical disc comprises a plurality of layers, from 2 to 100 or even more.
The exciting beam is focussed with an objective lens on a layer of the disc. When a fluorescent cell is illuminated by the exciting beam, a fluorescence signal is generated. This fluorescence signal has a wavelength, which is different from the wavelength of the exciting beam, and is detected by a detector unit. The detector unit comprises means for separating the fluorescence signal coming from the in-focus layer from the fluorescence signals coming from the out of focus layers. For example, a confocal pinhole is inserted in front of a photodiode in order to spatially block the fluorescence signal coming from the out-of-focus layers.
FIG. 1 illustrates a scanning apparatus for multilayer optical storage. Such a scanning apparatus comprises an exciting source 11, a collimator lens 12, a dichroic mirror 14, an objective lens 15, an imaging lens 18 and a detecting unit 19. This scanning apparatus is intended for reading a fluorescent multilayer carrier 16.
The exciting source generates an exciting beam 13. The collimator lens 12 is designed for providing a parallel exciting beam. The exciting beam 13 then reaches the dichroic mirror 14 and is directed to the objective lens 15, which focuses this exciting beam 13 on a layer of the carrier 16. The objective lens 15 can be moved up and down in order to focus the exciting beam 13 on the desired layer.
The exciting beam 13 interacts with the layers of the disc, which results in an excited radiation 17. This excited radiation passes through the dichroic mirror and reaches the imaging lens 18, which focuses the excited radiation 17 on the detecting unit 19.
The storage capacity of a layer depends on the area of the focussed exciting beam, i.e. the area of the spot created on a layer when the exciting beam 13 is focussed on this layer. In order to obtain a large storage capacity, a small focussed spot area is required.
Now, the focussed spot area is proportional to (λ/NA)2, where λ is the wavelength of the exciting beam 13 and NA is the numerical aperture of the exciting beam 13, i.e. the numerical aperture of the objective lens in this case. It thus seems judicious to use an objective lens having the largest possible numerical aperture, in order to obtain a large storage capacity. However, the useable numerical aperture for the objective lens is limited by the presence of aberrations, which occur for the parts of the exciting beam 13 passing through the outer pupil regions of the objective lens. These aberrations strongly increase with the numerical aperture of the objective lens. For example, the third order spherical aberrations increase with NA4. This is a drawback because the aberrations strongly affect the signal detected by the detecting unit 19, leading to an unreliable scanning apparatus. Actually, in multilayer storage, different layers at different positions within the carrier have to be accessed, leading to layer-dependent aberrations, especially spherical aberrations. It is not possible, as in single layer storage, to pre-compensate for this with a single lens.
For this reason, the useable objective lens numerical aperture is limited, in practice to roughly 0.6. Now, another consideration has to be taken into account. The geometric emission characteristic of the excited radiation is not identical with that of the exciting beam, i.e. in an isotropic case, the excited radiation propagates in all directions from the excited fluorescent cells. The excited radiation 17 collected on the objective lens 15 thus corresponds to a small part of the excited radiation induced by the exciting beam, and the smaller the numerical aperture of the objective lens 15, the smaller the part of the excited radiation collected on the objective lens 15.
Therefore, the signal detected by the detecting unit 19, corresponding to the excited radiation collected on the objective lens 15, is low with the useable objective lenses, which have a numerical aperture inferior to 0.6. A low detected signal is a drawback, because it leads to a low signal to noise ratio, and thus to a limited bandwidth and data rate of the scanning apparatus.